Metal-silicide-nitridation for stress reduction

ABSTRACT

A pellicle for a lithographic apparatus, the pellicle including nitridated metal silicide or nitridated silicon as well as a method of manufacturing the same. Also disclosed is the use of a nitridated metal silicide or nitridated silicon pellicle in a lithographic apparatus. Also disclosed is a pellicle for a lithographic apparatus including at least one compensating layer selected and configured to counteract changes in transmissivity of the pellicle upon exposure to EUV radiation as well as a method of controlling the transmissivity of a pellicle and a method of designing a pellicle.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority of EP application 17200069.7 which wasfiled on Nov. 6, 2017 and EP application 18179205.2 which was filed onJun. 22, 2018 and which are incorporated herein in its entirety byreference.

FIELD

The present invention relates to a pellicle for a lithographicapparatus, a method of manufacturing a pellicle for a lithographicapparatus, and a lithographic apparatus comprising a pellicle, as wellas uses therefor.

BACKGROUND

A lithographic apparatus is a machine constructed to apply a desiredpattern onto a substrate. A lithographic apparatus can be used, forexample, in the manufacture of integrated circuits (ICs). A lithographicapparatus may for example project a pattern from a patterning device(e.g. a mask) onto a layer of radiation-sensitive material (resist)provided on a substrate.

The wavelength of radiation used by a lithographic apparatus to projecta pattern onto a substrate determines the minimum size of features whichcan be formed on that substrate. A lithographic apparatus which uses EUVradiation, being electromagnetic radiation having a wavelength withinthe range 4-20 nm, may be used to form smaller features on a substratethan a conventional lithographic apparatus (which may for example useelectromagnetic radiation with a wavelength of 193 nm).

A lithographic apparatus includes a patterning device (e.g. a mask orreticle). Radiation is provided through or reflected off the patterningdevice to form an image on a substrate. A pellicle may be provided toprotect the patterning device from airborne particles and other forms ofcontamination. Contamination on the surface of the patterning device cancause manufacturing defects on the substrate.

Pellicles may also be provided for protecting optical components otherthan patterning devices. Pellicles may also be used to provide a passagefor lithographic radiation between regions of the lithography apparatuswhich are sealed from one another. Pellicles may also be used asfilters, such as spectral purity filters. Due to the sometimes harshenvironment inside a lithography apparatus, particularly an EUVlithography apparatus, pellicles are required to demonstrate excellentchemical and thermal stability.

Known pellicles may comprise, for example, a freestanding graphenemembrane, graphene derivative, including graphene halides, graphane,fullerene, carbon nanotube, or other carbon-based material. A maskassembly may include the pellicle which protects a patterning device(e.g. a mask) from particle contamination. The pellicle may be supportedby a pellicle frame, forming a pellicle assembly. The pellicle may beattached to the frame, for example, by gluing a pellicle border regionto the frame. The frame may be permanently or releasably attached to apatterning device. The freestanding graphene membrane may be formed byfloating a thin film of graphene on a liquid surface and scooping thethin film onto a silicon frame. The quality of membranes formed in thisway has been found to be variable and difficult to control. Furthermore,it is difficult to produce large graphene membranes reliably.

It has been found that the lifetime of known pellicles, such aspellicles comprising freestanding graphene membranes or othercarbon-based membranes, is limited.

It has also been found that known pellicles may be etched in anatmosphere which contains free radical species, such as H* and HO*, andthereby can degrade over time with use. Since pellicles are very thin,the reaction with the free radical species can weaken the pellicle andultimately cause it to fail. Alternative materials for use as pelliclesare therefore required.

In addition, it has been found that the transmissivity of a pellicle canvary over time. This affects the amount of radiation which is able topass through the pellicle and may therefore cause under-or over-exposureof the resist used in the lithographic apparatus. Also, if thetransmissivity decreases, this can lead to the pellicle operating at ahigher temperature than would otherwise be the case, which can lead todamage to the pellicle and a reduction in the lifespan of the pellicle.Alternative pellicles which are less susceptible to changes intransmissivity during use are therefore desirable.

Refractory metal silicides such as molybdenum disilicide, niobiumdisilicide, tantalum disilicide, and tungsten disilicide, have beeninvestigated for use as gate materials, ohmic contacts, and heatingelements due to their chemical and thermal stability and theirelectrical conductivity. Until the present invention, it has not beenpossible to use such materials as pellicles.

Metal silicide compounds may be used for transistor gates. These may beformed by depositing a layer of the metal silicide compound on a siliconwafer. The metal silicide layer or film may be deposited on the siliconby vapour deposition techniques, such as physical vapour deposition(PVD) or chemical vapour deposition (CVD). In the deposition step, arefractory metal having a high melting point, such as molybdenum, isdeposited on a silicon wafer, where it reacts to form a metal silicidelayer. The metal silicide layer may be provided with a sacrificial layerof silicon or silicon oxide in order to protect the metal silicidelayer. Such layered materials have been used in semiconductor devicesfor reducing the electrical resistance of silicon gate electrodes orsilicon wiring layers formed on a semiconductor substrate or source anddrain regions or diffused wiring layers formed in a major surface of asemiconductor substrate of single crystal silicon to as low as possible.

However, it has been found that such materials as are known in thesemiconductor industry for use in microelectronics are unsuitable foruse as pellicles. It will be appreciated that pellicles are much largerthan microelectronic chips and are subject to much harsher operatingconditions. In addition, the electronic properties of such materials areof primary importance when used in microelectronics, but it is thephysical properties which are more important when used as a pellicle.Further, it has not previously been possible to manufacture metalsilicide films which are larger than around 1 cm by 1 cm, and so knownmetal silicide films are unable to be used as pellicles.

In conventional manufacturing of freestanding metal silicide films, thefilm is heated to around 900° C. or more to allow the film to anneal.The annealing allows the metal silicide to find the lowest stress stateat that particular temperature and to increase the density of the metalsilicide film. When allowed to cool, the metal silicide shrinks morethan the silicon substrate (e.g. a silicon wafer) on which the metalsilicide film is grown, which results in high tensile stresses in themetal silicide layer after cooling. In order to recover the metalsilicide layer, the remainder of the wafer is etched away. The metalsilicide layer floats in the etchant and is able to be recovered.However, the high tensile stresses within the metal silicide film areretained.

Without wishing to be bound by scientific theory, it is believed thatmetal silicide films formed in this way are unable to be grown to morethan 1 cm by 1 cm area due to the stresses caused by the mismatchbetween the coefficients of thermal expansion of the silicon substrateand the metal silicide film. In particular, the metal silicide filmsexpand more than the silicon substrate when heated and shrinks more whencooled down, resulting in high tensile stresses in the metal silicidelayer.

It is necessary to anneal the metal silicide layer in order to densifythe metal silicide layer. If annealing is not done before the materialis used as a pellicle in the lithographic apparatus, then the materialwould densify and shrink when the material is heated during exposure inthe lithography apparatus. This would result in high tensile stresses inthe material and probable failure of the pellicle.

Another reason for exposing the pellicle film to high temperatures is toallow the deposition of high quality sacrificial oxides. The depositionof the sacrificial oxide layer is to allow the release of the ultrathinpellicle film. Pinholes in the sacrificial oxide layer are avoided bydeposition of the sacrificial oxides at high temperatures. As such, thedeposition of sacrificial oxides required for pellicle releaseintroduces high temperatures into the manufacturing process. Thesacrificial oxide may be provided by the decomposition of tetraethylorthosilicate (TEOS). At temperatures above around 600° C., TEOSdecomposes into silicon dioxide and diethyl ether.

In practice, a metal silicide layer is deposited on the surface of amonocrystalline silicon wafer. The wafer is then annealed, which may beachieved by heating to a temperature around 400 to 600° C., such as 500°C. The pellicle is then heated to a minimum temperature of around 725°C. or more, preferably a minimum of around 750° C., to allow thesacrificial oxides such as TEOS and thermal oxide to decompose andstabilise the metal silicide layer. These temperatures induce a largetensile stress of around 0.5 to 1.5 GPa, for example 1 GPa in the metalsilicide film. The silicon wafer is preferably a monocrystalline siliconwafer, but germanium wafers or wafers made from other materials that aresuitable for EUV transmission may also be used.

Whilst such films have good density, the tensile stress is too high andthey are therefore unable to be grown large enough to be used aspellicles in a lithographic apparatus and are otherwise unstable due tothe high internal stresses.

It is therefore desirable to provide a method for manufacturing apellicle which allows the manufacture of metal silicide films that arelarge and stable enough to be used as pellicles, preferably inlithographic apparatus, particularly EUV lithographic apparatus. It isalso desirable to provide a pellicle which is thermally and chemicallystable, and which is stronger than known metal silicide materials.

Whilst the present application generally refers to pellicles in thecontext of lithography apparatus, in particular EUV lithographyapparatus, the invention is not limited to only pellicles andlithography apparatus, and it is appreciated that the subject matter ofthe present invention may be used in any other suitable apparatus orcircumstances.

For example, the methods of the present invention may equally be appliedto spectral purity filters. EUV sources, such as those which generateEUV radiation using a plasma, in practice do not only emit desired‘in-band’ EUV radiation, but also undesirable (out-of-band) radiation.This out-of-band radiation is most notably in the deep UV (DUV)radiation range (from 100 to 400 nm). Moreover, in the case of some EUVsources, for example laser produced plasma EUV sources, the radiationfrom the laser, usually at 10.6 microns, may also form a significantsource of undesirable (out-of-band) infrared (IR) radiation.

In a lithographic apparatus, spectral purity is desired for severalreasons. One reason is that resist is sensitive to out of-bandwavelengths of radiation, and thus the image quality of exposurepatterns applied to the resist may be deteriorated if the resist isexposed to such out-of-band radiation. Furthermore, out-of-band infraredradiation, for example the 10.6 micron radiation in some laser producedplasma sources, leads to unwanted and unnecessary heating of thepatterning device, substrate, and optics within the lithographicapparatus. Such heating may lead to damage of these elements,degradation in their lifetime, and/or defects or distortions in patternsprojected onto and applied to a resist-coated substrate.

A typical spectral purity filter may be formed, for example, from asilicon membrane that is coated with a reflective metal, such asmolybdenum or ruthenium. In use, a typical spectral purity filter mightbe subjected to a high heat load from, for example, incident infraredand EUV radiation. The heat load might result in the temperature of thespectral purity filter being above 800° C., which results in eventualdelamination of the coating. Delamination and degradation of the siliconmembrane is accelerated by the presence of hydrogen, which is often usedas a gas in the environment in which the spectral purity filter is usedin order to suppress debris (e.g. molecular outgassing from resists, orparticles debris or the like), from entering or leaving certain parts ofthe lithographic apparatus.

Thus, a metal silicide film according to the invention may be used as aspectral purity filter to filter out undesired radiation, and it mayalso be used as a pellicle to protect a lithographic mask from beingcontaminated with particles. Therefore, reference in the presentapplication to a ‘pellicle’ is also reference to a ‘spectral purityfilter’ (the terms may be interchanged). Although reference is primarilymade to pellicles in the present application, all of the features couldequally be applied to spectral purity filters. It is understood thatspectral purity filters are a type of pellicle.

In a lithographic apparatus (and/or method) it is desirable to minimisethe losses in intensity of radiation which is being used to apply apattern to a resist coated substrate. One reason for this is that,ideally, as much radiation as possible should be available for applyinga pattern to a substrate, for instance to reduce the exposure time andincrease throughput. At the same time, it is desirable to minimise theamount of undesirable radiation (e.g. out-of-band) radiation that ispassing through the lithographic apparatus and which is incident uponthe substrate. Furthermore, it is desirable to ensure that a pellicleused in a lithographic method or apparatus has an adequate lifetime, anddoes not degrade rapidly over time as a consequence of the high heatload to which the pellicle may be exposed, and/or the hydrogen (or thelike, such as free radical species including H* and HO*) to which thepellicle may be exposed. It is therefore desirable to provide animproved (or alternative) pellicle, and for example a pellicle suitablefor use in a lithographic apparatus and/or method.

Furthermore, whilst the present application generally refers tomolybdenum disilicide pellicles, it will be appreciated that anysuitable metal silicide materials may be used. For example, the pelliclemay comprise zirconium, niobium, lanthanum, yttrium, and/or berylliumdisilicides. In addition, the embodiments of the invention relating to apellicle having at least one sacrificial layer which is selected andconfigured to counteract changes in the transmissivity of the pellicleupon exposure to EUV radiation and the associated methods may be appliedto pellicles comprising nitridated metal silicide or nitridated silicon,or may be applied to any other type of pellicle.

SUMMARY

The present invention has been made in consideration of theaforementioned problems with known pellicles and known methods ofproducing and designing pellicles.

According to a first aspect of the present invention, there is provideda pellicle for a lithographic apparatus, wherein the pellicle comprisesnitridated metal silicide or nitridated silicon.

It has been surprisingly found that the addition of nitrogen to themetal silicide results in numerous advantages over metal silicide orsilicon layers, wafers, films or similar which do not comprise nitrogen.These advantages make it possible to provide a pellicle comprising ametal silicide film, which has not previously been possible. The metalsilicide substrate may be a molybdenum silicide or a zirconium silicidesubstrate.

By nitriding a metal silicide substrate, nitrogen is able to react withthe metal silicide and form metal-silicide-nitride on a pelliclesubstrate. The metal-silicide-nitride layer may be formed on a siliconsubstrate, which may be a silicon wafer. Similarly, it is possible todope pure silicon with nitrogen in order to improve the strength of apolysilicon pellicle. In this case, the substrate is substantially puresilicon.

Firstly, the addition of nitrogen has been surprisingly found to keepthe film more amorphous than is the case without the additionalnitrogen. This results in increased strength, resistance to heat, andresistance to mechanical loads, which is demonstrated by improvedtensile strength.

In addition, the addition of nitrogen keeps the metal silicide film in amore compressive state during the annealing process. As such, as themetal silicide nitride film is cooled to room temperature, the filmcontracts to a lower extent than would otherwise be the case and thereis consequently a lower residual tensile stress within the film at roomtemperature. Further, the film will be kept at a zero-state density andwithout the stress accumulation during high power exposures, which leadto high temperatures of around 450° C. to 600° C. In particular, thepellicle will not shrink when heated as the addition of nitrogen causesthe pellicle to already be dense when it is deposited, thereby becomingmore resistant to density changes during use.

Further surprising advantages due to the inclusion of nitrogen are thereduction in oxidation of the metal-silicide and the reduction in nativeoxide thickness. The reduction in susceptibility to oxidation improvesthe chemical and thermal stability of the metal silicide and thereduction in the native oxide thickness reduces the stresses on themetal silicide. Without wishing to be bound by scientific theory, it isbelieved that the native oxide layer introduces compressive stress andtherefore exerts a tensile force on the pellicle film, thus weakeningthe film. The reduction in the thickness of the native oxide layer isbelieved to reduce the tensile stress. The reduction in native oxidethickness also aids in improving EUV transmission. It is important thatas much of the EUV radiation available is able to pass through thepellicle without being absorbed in order to avoid reducing the power ofthe EUV radiation and thereby decreasing the efficiency of the apparatusas a whole and reducing the throughput of the apparatus.

The addition of nitrogen to the metal silicide layer has also been foundto reduce the coefficient of linear thermal expansion of the material.This leads to lower tensile stresses in the material caused by changesin temperature, again leading to reduced tensile stresses and therebyallowing a larger film to be produced.

Similar advantages are also seen with a nitrided silicon pellicle.

Preferably, the metal silicide nitride has the formulaM_(x)(Si)_(y)N_(z), wherein x≤y≤2x, and 0<z≤x. The exact amount ofnitrogen added can be adjusted depending on the nature of the metal inthe compound as well as the operating conditions intended for thepellicle. For example, it is possible to include a higher amount ofnitrogen in a zirconium silicide compound compared to a molybdenumsilicide compound since zirconium is more transparent to EUV radiationthan molybdenum and so, although an increase in the amount of nitrogenreduces EUV transmission, this is balanced by the improvedtransmissivity of the zirconium.

Accordingly, there is a larger atomic concentration of silicon thanmetal in the metal-silicide-nitride film. Preferably, the atomicconcentration of silicon is around twice that of the metal, namely y=2x.It will be appreciated that non-stoichiometric values are possible. Forexample, the value of y may be any number between x and 2x, including xand 2x.

Since the presence of nitrogen is necessary in order to provide themetal silicide film with the desired physical characteristics, the valueof z is greater than zero. Since the addition of nitrogen reduces theelectrical conductivity of the metal silicide material at highconcentrations, and also since the EUV transmissivity decreases at highnitrogen concentrations, it is preferable to keep the nitrogen contentas low as possible in the metal silicide layer, but high enough todemonstrate the aforementioned advantages. It has been found that havingx as less than one does not adversely affect EUV transmissivity, butprovides the mechanical advantages described herein. As such, the valueof z is less than or equal to the value of x. Preferably, the value of zis 1 or less.

Preferably, the nitridated silicon has the formula SiN_(a), wherein0.01≤a≤1. Preferably, a≤0.5, and more preferably a≤0.1. Whilst nitrogendoped silicon is known for use in microelectronics, the highest amountof nitrogen in doped silicon is around one nitrogen atom per tenthousand silicon atoms, i.e. 0.01 atomic %. In the present context, thiswould mean that the maximum value of ‘a’ was 0.0001. In lightly dopedsilicon, the value of ‘a’ would be orders of magnitude smaller. Further,in silicon nitride (Si₃N₄) there is a greater proportion of nitrogenatoms than silicon atoms, i.e. ‘a’>1. As such, the formula of thepellicle of the present invention falls outside that used inmicroelectronics and also when silicon nitride is used as a bulkmaterial in, for example, bearings or turbochargers.

The formula of the material comprising the pellicle does not have to bestoichiometric. The formula is to be interpreted as having been reducedto the lowest common denominator and/or shown in a shortened format. Forexample, where the formula of the film is Mo₂Si₄N₁, this can also beexpressed as MoSi₂N_(0.5). In another example, where the formula of thefilm is Zr₃Si₆N₁, this may also be expressed as ZrSi₂N_(0.33). Indeed,since the silicon is only being partially nitridated, the formula willnot be stoichiometric.

Preferably, the metal (M) is selected from the group comprising Ce, Pr,Sc, Eu, Nd, Ti, V, Cr, Zr, Nb, Mo, Ru, Rh, La, Y, and Be. Preferably,the metal (M) is Mo or Zr or Be.

Exemplary compositions of the pellicle are ZrSi₂N, MoSi₂N, LaSi₂N, andYSi₂N. In each of these examples, x=1, y=2, and z=1.

Other exemplary compositions are Mo₂Si₄N, Zr₂Si₄N, Mo₃Si₆N, and Zr₃Si₆N.As can be seen from these examples, the atomic concentration of nitrogenis less than the atomic concentration of the metal. Preferably, theatomic concentration of nitrogen is less than around 25% of the totalatomic concentration of metal, silicon, and nitrogen. As such, the metaland silicon preferably comprise more than around 75% of the total numberof metal, silicon and nitrogen atoms in the metal silicide film. Inother terms, more than around 75% of the total atoms in themetal-silicide-nitride pellicle are metal or silicon atoms, with theremaining around 25% being nitrogen atoms.

The atomic concentration of nitrogen may be less than around 20%, lessthan around 15%, less than around 10%, less than around 5%, or less thanaround 1%.

The pellicle may further comprise at least one capping layer. Thepellicle may comprise a capping layer on each side of themetal-silicide-nitride or nitridated silicon film. Themetal-silicide-nitride or nitridated silicon film may be from 10 toaround 40 nm thick, preferably from around 15 to around 30 nm. The atleast one capping layer may be around 0.1 to around 10 nm thick,preferably around 1 to around 5 nm thick. The capping layer may compriseany suitable capping material. Suitable capping materials are ones whichare thermally and chemically stable in the environment of an EUVlithography apparatus and which do not significantly inhibit EUVtransmission through the pellicle. The capping layer must also becompatible with the pellicle so that it is able to adhere to thenitridated metal silicide or silicon. Suitable coating materials includeruthenium Ru, boron B, metal borides, carbon boride B4C, boron nitrideBN, or similar.

The capping material can be applied using any suitable method, such as,for example, chemical vapour deposition or sputtering.

In practice, the M_(x)Si_(y)N_(z) may be manufactured at ambienttemperatures on a wafer. The wafer may then be etched in a suitableliquid and the pellicle film may be lifted out of the liquid onto aframe. In this case, the addition of the nitrogen primarily increasesthe density of the film and therefore heat resistance. This may be usedto produce EUV filters for various applications. M_(x)Si_(y)N_(z) mayalso be manufactured using a CMOS (Complementary Metal OxideSemiconductor) process which incorporates a high temperature anneal, andhigh temperature deposition of sacrificial oxides. The lower coefficientof thermal expansion due to the addition of nitrogen and enhancedresistance to structural changes due to the addition of nitrogen mainlyserve to reduce stress and allow the manufacture of full-size pellicles.

According to a second aspect described herein, there is provided amethod of manufacturing a pellicle for a lithographic apparatus, saidmethod comprising nitriding a metal silicide or silicon substrate.

The nitriding of the metal silicide or silicon is effected by sputteringthe metal silicide or silicon substrate with a plasma. The sputteringmay be reactive sputtering. The plasma may be any suitable plasma. Theplasma preferably comprises nitrogen. Preferably, the plasma comprises amixture of argon and nitrogen gas. The argon gas is included in order toprovide an inert atmosphere. Argon is preferably used as it is cheaperthan other noble gases, but other noble gases could be used.

The ratio of argon to nitrogen may be varied. Having a larger proportionof nitrogen in the gas mixture will result in a greater amount ofnitrogen being incorporated into the metal silicide film. For example,where the nitrogen flow ratio, which is calculated as the amount ofnitrogen divided by the amount of nitrogen plus argon, was around 10%,this resulted in an atomic concentration of around 18% of nitrogen inthe metal silicide nitride film. When the nitrogen flow ratio was around40%, the resulting atomic concentration of nitrogen of around 42% in themetal silicide nitride film. Similarly, as the nitrogen flow ration wasincreased from 10% to 40%, there was a corresponding decline in theatomic concentration of oxygen from around 34% to around 15%,demonstrating the reduction in thickness of the native oxide layer. Assuch, the ratio of argon to nitrogen may be varied depending on thedegree of nitridation required.

The metal forming the metal silicide may be selected from the groupcomprising Ce, Pr, Sc, Eu, Nd, Ti, V, Cr, Zr, Nb, Mo, Ru, Rh, La, Y, andBe. Of this group, molybdenum, zirconium, and beryllium are thepreferred elements. Molybdenum is most preferred.

Due to the limitations of known methods for producing pellicles, untilnow, there has been no suitable way of making a pellicle comprisingmetal silicide.

Nitridated metal silicide materials have previously only been used toform gates in semiconductor transistors, which are degrees of magnitudesmaller than pellicles and do not have to withstand the harsh thermaland chemical environment of a lithography apparatus, particularly an EUVapparatus.

Thus, according to a third aspect of the present invention, there isprovided a pellicle for a lithographic apparatus obtainable or obtainedby the method according to the second aspect of the present invention.

According to a fourth aspect of the present invention, there is providedthe use of a pellicle manufactured by the method according to the secondaspect of the present invention or a pellicle according to the firstaspect of the present invention in a lithographic apparatus.

Since it has not previously been possible to manufacture a metalsilicide pellicle having the required physical characteristics requiredfor use as a pellicle, it has not been possible to use such a pelliclein a lithographic apparatus. Furthermore, it has been surprisinglyrealised that nitridating silicon can result in an increase in strengthof the silicon pellicle.

According to a fifth aspect of the present invention, there is providedthe use of reactive sputtering to manufacture a pellicle according tothe first aspect of the present invention.

According to a sixth aspect of the present invention, there is providedan assembly for a lithographic apparatus comprising a pellicle accordingto the any of the aforementioned aspects of the present invention, aframe for supporting the pellicle and a patterning device attached tothe frame.

According to a seventh aspect of the present invention, there isprovided a pellicle for a lithographic apparatus comprising at least onecompensating layer selected and configured to counteract changes in thetransmissivity of the pellicle upon exposure to EUV radiation.

It has been found that the transmissivity of a pellicle changes uponexposure to EUV radiation. The change may be irreversible. The changemay be rapid upon exposure to EUV or the extent of the change may bedependent on the length of time which the pellicle is exposed to the EUVradiation as well as the power level used. The change in transmissivitymay be caused by a number of factors. For example, certain materialsused in pellicles oxidize when subjected to the sometimes harshtemperatures in an EUV lithography apparatus. The oxides produced duringuse of the pellicle may be volatile, such as silicon oxide or carbonmonoxide/dioxide. As such, these gaseous oxides may leave the pellicleand the pellicle will reduce in thickness over time which will lead toan increase in the transmissivity of the pellicle. In contrast, certainoxides will remain on the pellicle and these may have lowertransmissivity than the unoxidised form of the material. The change intransmissivity may also be caused by erosion or etching of material fromthe pellicle during use with or without oxidation.

Previously, changes in transmissivity of a pellicle during use and thelimitation to the lifetime of the pellicle have either been accepted asinevitable or attempts have been made to prevent oxidation of materialscomprising the pellicle by including oxidation-resistant materials inthe pellicle. The compensating layer may be sacrificed by being removedfrom the pellicle or by being physically changed whilst remaining aspart of the pellicle. As such, the compensating layer may be asacrificial layer.

The invention according to the seventh aspect of the present inventiontakes a different approach to the prior art by seeking to balance thechange in transmissivity of one material in the pellicle by includinganother material which displays the opposite change in transmissivityupon exposure to EUV radiation.

Preferably, the at least one compensating layer comprises a materialwhich alters upon exposure to EUV radiation to increase or decrease thetransmissivity of the at least one compensating layer.

The at least one compensating layer is configured so that the change intransmissivity of the at least one compensating layer mirrors the changein transmissivity of the pellicle so that the overall transmissivity ofthe pellicle is substantially constant. It will be appreciated that thetransmissivity of the pellicle will not be completely constantindefinitely as the compensating layer will ultimately be completelysacrificed. Even so, the presence of the compensating layer which isselected and configured to counteract the change in transmissivity ofthe pellicle will extend the operating lifetime of the pellicle.

The compensating layer may comprises one or more of silicon dioxide,silicon, silicon nitride, silicon carbide, carbon, boron carbide,ruthenium dioxide, boron, zirconium boride, and molybdenum. Thecompensating layer may comprise any material which is able to withstandthe conditions within an EUV lithography apparatus and which alter theirtransmissivity upon exposure to EUV radiation.

Boron, zirconium boride, and molybdenum have been found to showdecreased EUV transmissivity upon exposure to EUV radiation. Withoutwishing to be bound by scientific theory and for the sake of an example,boron may be oxidized when exposed to the operating conditions of an EUVlithography apparatus to yield boron oxide. Boron oxide has much greaterEUV absorption than boron, so the creation of boron oxide on a pelliclewill result in a lower transmissivity. As such, these materials may beused to counteract a gain in EUV transmissivity.

On the other hand, silicon dioxide, silicon, silicon nitride, siliconcarbide, carbon, boron carbide, and ruthenium dioxide have been found toshow increased EUV transmissivity upon exposure to EUV radiation. Again,without wishing to be bound by scientific theory and for the sake of anexample, carbon may be oxidized to form carbon monoxide or carbondioxide. Both of these compounds are gaseous under the operatingconditions of an EUV lithography apparatus and so leave the pellicle.Over time, the reduction in material will lead to an increase in thetransmissivity of the pellicle.

Therefore, it is possible to provide a compensating layer on a pellicleto take account of the tendency of the materials of the pellicle toincrease or decrease EUV transmissivity on use. The thickness of thecompensating layer may be adjusted so that it provides a sufficientlythick oxide layer to counteract or compensate for the loss of materialof the pellicle, or so that it is thick enough to provide enoughmaterial which is able to leave the pellicle to counteract the decreasein transmissivity of the pellicle over the lifetime of the pellicle.

According to an eighth aspect of the present invention, there isprovided a method of controlling changes in the transmissivity of an EUVpellicle, the method comprising the steps of: providing at least onelayer which has increased transmissivity upon exposure to EUV radiationand/or at least one layer which has decreased transmissivity uponexposure to EUV radiation.

It has been surprisingly realised that it is possible to control thetransmissivity of a pellicle during use in an EUV lithography apparatusby providing at least one layer, which may be referred to as acompensating layer, that has increased or decreased (as appropriate)transmissivity upon exposure to EUV radiation and/or the operatingconditions of an EUV lithography apparatus. Previously, attempts weremade to prevent physical changes in the pellicle, such as oxidation oretching, in order to prevent degradation of the pellicle. In contrast,the method according to the eighth aspect of the present inventionsolves the problem of the varying transmissivity of a pellicle byproviding a compensating layer.

According to a ninth aspect of the present invention, there is provideda method of designing a pellicle for a lithography apparatus, the methodcomprising the steps of: measuring the change in transmissivity of apellicle upon exposure to EUV radiation, and using the measured changein transmissivity to select one or more materials having a change intransmissivity upon exposure to EUV radiation which most closely mirrorsthe change in transmissivity of the pellicle for inclusion in an updatedpellicle. Once selected, the material comprising the compensating layermay be added to a pellicle, thus forming a pellicle comprising theidentified material.

This method allows a pellicle to be produced which has a more stabletransmissivity upon use than previous pellicles. The change intransmissivity of a pellicle or materials which may serve as acompensating layer can be measured routinely by known techniques andapparatus. Therefore, it is possible to measure how the transmissivityof the pellicle changes over time and then match it to a material whichdisplays the opposite change, so that when the pellicle and the materialare combined to form an updated pellicle, the two will cancel oneanother out and the transmissivity of the pellicle will be more constantthan the original pellicle.

The change in transmissivity of the pellicle upon exposure to EUVradiation may be measured over a preselected length of time. Thepreselected length of time is approximately the same order of time thatthe pellicle is in use in an EUV lithography apparatus.

Since a pellicle will preferably withstand use in an EUV lithographymachine for at least a day and preferably longer, the measurement of thechange in transmissivity will be measured over a time period which is ofthe same order of magnitude as the expected life of the pellicle. Inthis way, the change in transmissivity of the pellicle over time can bedetermined and the sacrificial compensating layer can be more accuratelyselected. For example, the preselected period of time may be between 1and 24 hours, but may be up to seven days, if required.

The change in transmissivity of the pellicle upon exposure to EUVradiation may be measured at a preselected temperature and/or powerlevel. The temperature and/or power level may be approximately the sametemperature and/or power level to which the pellicle is exposed duringuse in an EUV lithography apparatus.

In order to provide a suitable model of the change in transmissivity ofa pellicle, it is necessary to subject the pellicle to the conditions inwhich it will be used. This allows the most appropriate compensatinglayer to be selected for inclusion in the updated pellicle. For example,the pellicle may be tested at temperatures from around 400° C. up toaround 900° C. For example, the pellicle may be tested at power levelsof from around 50 W to around 500 W.

Once an updated pellicle comprising the sacrificial compensating layerhas been provided, it may be subject to further testing to determine howthe transmissivity of the updated pellicle changes over time under theconditions inside an EUV lithography apparatus. Based on this furtherinformation, the updated pellicle can then be refined and improved byadjusting the compensating layer, such as, for example, by altering thethickness, position, and/or composition of the compensating layer. Thisrefinement can be repeated until an optimized pellicle is reached.

According to a tenth aspect of the present invention, there is provideda pellicle designed according to the methods of the eighth or ninthaspects of the present invention.

A pellicle according to the tenth aspect of the present invention willdemonstrate improved stability with respect to EUV transmissivity thanother pellicles.

According to an eleventh aspect of the present invention, there isprovided a method of manufacturing a membrane assembly for EUVlithography, the method comprising: providing a stack comprising: aplanar substrate, wherein the planar substrate comprises an inner regionand a border region around the inner region; at least one membranelayer; an oxide layer between the planar substrate and the at least onemembrane layer; and at least one further layer between the planarsubstrate and the at least one membrane layer; and selectively removingthe inner region of the planar substrate, such that the membraneassembly comprises: a membrane formed at least from the at least onemembrane layer; and a border holding the at least one membrane layer,the border comprising at least a portion of the planar substrate, the atleast one further layer, and the oxide layer situated between the borderand the at least one membrane layer.

It has been noted that some membrane layers are susceptible to weakeningduring manufacture due to over etching. Different etching processes etchdifferent materials at different rates. As such, in certain etchingprocesses, one material may be etched at a different rate to anothermaterial. Furthermore, during etching, it has been found that certainportions of a given layer may be etched at a different rate to otherportions of the same layer. In particular, the edge portions of a givenlayer are generally etched at a faster rate than the central portion ofthe given layer. Without wishing to be bound by scientific theory, it isbelieved that the etchant fluid may become diluted by the etch productmore in the region of the central portion of a given layer than at theedge portions of the same layer. As such, the rate of etching is reducedadjacent the central portion of a given layer when compared to the edgeportions of the same layer, which results in a non-uniform etch. Thedegree of non-uniformity defines the minimum thickness of the layersbeing etched and is ultimately translated into a lack of uniformity inthe ultimate membrane assembly. This non-uniformity can weaken themembrane layer and result in premature failure of the membrane layer inuse or require more frequent replacement of a pellicle comprising thestack than would otherwise be the case.

According to the method of the eleventh aspect of the present invention,the presence of at least one further layer between the planar substrateand the at least one membrane layer may serve to reduce or overcome theproblem of over-etching. The at least one further layer is preferablyetched at a considerably slower rate than the oxide layer. Preferably,the at least one further layer is substantially resistant to the etchantused to etch the oxide layer. As such, during a step of bulk etching ofthe planar substrate, the etching process proceeds to etch away theinner region of the planar substrate until it reaches the oxide layer,which may be referred to as a buried oxide layer. The etchant used toetch away the inner region of the planar substrate may be atetramethylammonium hydroxide (TMAH) based etchant or other suitableetchant as known in the art that selectively etches silicon over siliconoxide. The oxide layer is substantially resistant to the etchant used toetch away the inner region of the planar substrate and therefore theetching process is stopped or slowed down considerably upon reaching theburied oxide layer. Since the etchant will not etch away or only veryslowly etch away the buried oxide layer, means that there is less riskof over etching of the buried oxide layer. Following this, an etchantwhich is able to etch away the buried oxide layer is used to remove atleast a portion of the buried oxide layer. Suitable etchants includebuffered oxide etchants (BOE), as are known in the art. The etchant usedetches the at least one further layer more slowly than the oxide layerand so any over etching of the buried oxide layer does not translateinto the at least one further layer. Since the buried oxide layer isthin, only a short etch is required to remove the buried oxide layer,which reduces the likelihood of the overlying at least one further layerbeing unevenly etched. A second etch step using TMAH etchant or othersuitable etchant which selectively etches silicon over silicon oxide maythen be used to remove the at least one further layer. Again, since theoxide layer which overlies the at least one further layer is resistantto the etchant used to etch away the at least one further layer, therisk of over etching is reduced and the resulting stack comprises a moreuniform oxide layer on the lower face of the at least one membranelayer.

A further advantage of the eleventh aspect of the present invention isthat it allows the buried oxide layer between the planar substrate andthe at least one membrane layer to be thinner. This reduces the tendencyof the membrane assembly to wrinkle, which can weaken the membraneassembly, since oxide layers comprise compressive stresses and thereforehaving a thinner oxide layer reduces the compressive stresses within thestack.

Preferably, the at least one membrane layer comprises molybdenum siliconnitride, although it will be appreciate that the present invention canbe applied to any membrane layer, such as, for example, pSi. The atleast one membrane layer can be any of the membrane layers described inrespect of any aspect of the present invention. For example, themembrane layer may comprise nitridated metal silicide or silicon.Molybdenum silicon nitride is sensitive to over etching using bufferedoxide etch (BOE), which comprises HF. Again, without wishing to belimited by scientific theory, it is believed that when overlying silicondioxide sacrificial layers are removed that the silicon nitride in themolybdenum silicon nitride is also etched, thereby creating notcheswhich weaken the layer. If the etching step is continued for too long,the entire layer could be damaged or destroyed. The present inventionserves to overcome this problem.

Preferably, the at least one further layer comprises silicon.Preferably, the at least one further layer comprises cSi or pSi or aSi.Silicon is etched faster than silicon oxide in TMAH etchant, whereassilicon oxide is etched faster than silicon in BOE. As such, it ispossible to selectively remove either a silicon layer or a silicon oxidelayer without etching an overlying silicon oxide layer or a siliconlayer, respectively.

There may be a further oxide layer, which may be a thermal oxide layerbetween the at least one further layer and the at least one membranelayer. As such, the order of the layers in the stack starting from thetop may be a membrane layer, a thermal oxide layer, a silicon layer, aburied oxide layer, and a planar substrate. The membrane layer may becapped with a layer of tetraethyl orthosilicate (TEOS), which may beconverted to a silicon oxide layer.

The planar substrate may comprise silicon. Silicon is awell-characterized material that is able to withstand the harshenvironment within a lithographic apparatus in use.

The step of removing the inner region of the planar substrate maycomprise etching using TMAH etchant. The stack may be exposed to theetchant until the etchant reaches the buried oxide layer.

A different etchant, for example BOE, may then be used to remove theburied oxide layer. The different etchant may be used until the etchantreaches the at least one further layer, which may comprise silicon.

A further etching step using TMAH etchant may then be used to etch awaythe at least one further layer. The etchant may be used until theetchant reaches the thermal oxide layer.

According to a twelfth aspect of the present invention, there isprovided a membrane assembly for EUV lithography, the membrane assemblycomprising: a membrane formed from at least one layer comprisingmolybdenum silicon nitride; and a border holding the membrane; whereinthe border region is formed from a planar substrate comprising an innerregion and a border region around the inner region, wherein the borderis formed by selectively removing the inner region of the planarsubstrate, wherein the assembly comprises a buried oxide layer, asilicon layer, and a thermal oxide layer between the border and themembrane.

The membrane assembly according to the twelfth aspect of the presentinvention comprises a thermal oxide layer which is thinner than thethermal oxide layer of other assemblies. Since the thermal oxide iscompressive, this can cause the membrane layer to wrinkle. By having athinner oxide layer, the compressive forces are reduced and thewrinkling of the membrane is also reduced. In addition, the etching ofthe thermal oxide is more uniform, which results in more uniformtransmission of radiation through the assembly.

Preferably, the planar substrate comprises silicon.

The assembly manufactured in accordance with the eleventh aspect of thepresent invention or according to the twelfth aspect of the presentinvention may be used as a pellicle, preferably in an EUV lithographyapparatus.

According to a thirteenth aspect of the present invention, there isprovided a method of preparing a stack comprising the steps of:providing a planar substrate, a membrane layer, and a tetraethylorthosilicate layer and annealing the stack, wherein the tetraethylorthosilicate layer includes boron such that at least a portion of theboron from the tetraethyl orthosilicate layer diffuses into the membranelayer during annealing.

Membrane layers, such as those comprising pSi and molybdenum siliconnitride are susceptible to over-etching, which can reduce the strengthof the layer and result in premature failure. It is desirable to preventover etching and this may be achieved by adding additional sacrificiallayers which serve as etch homogenization layers, as described above.Alternatively or additionally, it has been surprisingly found that it ispossible to make such membrane layers more resistant to etching byadding boron to the layers. It has been found that the addition of boronto silicon reduces the etch rate in TMAH by a factor of around 100.Without wishing to be bound by scientific theory, it is believed thatthe boron preferentially sits at the grain boundaries in the membranelayer. It is also believed that pellicles are especially sensitive toetchant at the grain boundaries and so the presence of boron at thegrain boundaries is believed to be the reason why the resulting filmsare more etch resistant.

It has been found that the addition of boron to the TEOS layer andsubsequent annealing causes boron to diffuse into the membrane layer.For molybdenum silicide and molybdenum silicon nitride membranes, thishas been found to increase the consistency in the physical properties ofmembrane assemblies made from stacks according to this aspect of thepresent invention, namely there are fewer weak assemblies. In addition,it is possible to produce larger membrane assemblies and the resultingmembrane assemblies perform in heat loading tests similarly to membraneassemblies which do not include boron.

In addition, pSi layers manufactured according to this method are atleast around 50% stronger than similar layers not including boron.Indeed, the samples tested did not fail at the limit of the testingapparatus (3 GPa), so the exact limit of the increase in strength hasyet to be defined. In addition, there is no reduction in the EUVtransmissivity of such membranes comprising boron. In addition, theemissivity of pSi membranes manufactured according to this method ismuch higher than for pSi membranes which do not include boron. Thisincreased emissivity is beneficial as it makes the performance of anymetallic caps less critical and may even allow the emissive metallic capto be eliminated.

The planar substrate may comprise any suitable material. Preferably, theplanar substrate comprises silicon.

The membrane layer may comprise any suitable material. Preferably, themembrane layer comprises at least one of silicon, molybdenum silicideand molybdenum silicon nitride.

The annealing may be carried out at any suitable temperature. Thetemperature at which TEOS is annealed is known in the art. Preferably,the annealing is carried out at a temperature of from around 400 toaround 1000° C. For example, the annealing may take place at 600° C.,700° C., 800° C., or 900° C., as well as intermediate temperatures. Theannealing may take place at a constant temperature or may take place ata variable temperature.

The tetraethyl orthosilicate layer may comprise from about 0.1 to about15 wt % boron, preferably from about 2 to about 10 wt % boron, and morepreferably from about 4 to about 8 wt % boron.

The TEOS layer may be provided by chemical vapour deposition or anyother suitable technique.

According to a fourteenth aspect of the present invention, there isprovided a stack comprising a planar substrate and a membrane layer,wherein the membrane layer is doped with boron.

The planar substrate may comprise silicon.

The membrane layer may at least partially surround the planar substrate.The membrane layer may comprise at least one of silicon, molybdenumsilicide, molybdenum silicon nitride or any other membrane layermaterial described herein.

The stack may further comprise a thermal oxide layer between the planarsubstrate and the membrane layer.

The stack may further comprise a boron-containing TEOS layer at leastpartially surrounding the membrane layer. The boron-containing TEOSlayer is preferably in contact with the membrane layer to allow theboron atoms to diffuse into the membrane layer.

The stack manufactured according to the method of the thirteenth aspectof the present invention or the stack according to the fourteenth aspectof the present invention may be used in any of the other methodsdescribed herein or in the manufacture of an assembly according to anyaspect of the present invention. For example, the stack according to thefourteenth aspect of the present invention may be used in the method ofthe twelfth aspect of the present invention. Boron doping of a membranelayer is applicable to all aspects of the present invention.

As detailed above, the features described in respect of any of theaspects may be combined with the features described in respect of any ofthe other aspects of the present invention. For example, the features ofthe pellicle according to the second aspect of the present invention maybe combined with the features of the first, third, fourth, and/or fifthaspects of the present invention. In addition, the pellicles accordingto the second aspect of the present invention may be designed by themethod according to the ninth aspect of the present invention. Allcombinations of aspects of the present invention may be combined withone another, except where the features of the aspects of the inventionare mutually exclusive.

In summary, the methods of the present invention allow for themanufacture of a pellicle, in particular a molybdenum silicide pellicleor a silicon pellicle, which has been nitridated in order to improve itsphysical characteristics. The resulting pellicle is suitable for use inlithographic apparatus, such as, for example, an EUV lithographyapparatus. It has not been previously possible to manufacture such apellicle. The pellicles manufactured according to the methods of thepresent invention are able to resist the high temperatures achieved whenthe pellicle is in use and also resist attack by free radical species orother reactive species. The methods of the present invention allow forpellicles with a surface area of up to 10 cm by 14 cm in size. Themethods of the present invention also allow for the design andmanufacture of a pellicle which displays improved stability in thecontext of EUV transmissivity upon use in an EUV lithography machine. Apellicle comprising a compensating layer would extend the life of apellicle and would reduce the change in transmissivity of the pellicleover its lifetime and therefore allow a consistent number of wafers tobe imaged in a given time period.

The present invention will now be described with reference to an EUVlithography apparatus. However, it will be appreciated that the presentinvention is not limited to pellicles and is equally applicable tospectral purity filters.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings, in which:

FIG. 1 depicts a lithographic system comprising a lithographic apparatusand a radiation source according to an embodiment of the invention;

FIG. 2 depicts a schematic view of a pellicle according to the presentinvention and manufactured by the methods of the present invention,

FIG. 3 depicts a schematic of the steps used in selecting the correctcompensating layer for a given pellicle,

FIG. 4 depicts a schematic cross-section of a membrane assemblymanufactured according to an existing method,

FIGS. 5a to 5c depict a schematic cross-section of a membrane assemblymanufactured according to a method according to the eleventh aspect ofthe present invention, and

FIG. 6 depicts a schematic of a method according to the thirteenthaspect of the present invention.

DETAILED DESCRIPTION

FIG. 1 shows a lithographic system including a pellicle 15 according tothe first aspect of the present invention or manufactured according tothe methods of the second aspect of the present invention according toone embodiment of the invention. The lithographic system comprises aradiation source SO and a lithographic apparatus LA. The radiationsource SO is configured to generate an extreme ultraviolet (EUV)radiation beam B. The lithographic apparatus LA comprises anillumination system IL, a support structure MT configured to support apatterning device MA (e.g. a mask), a projection system PS and asubstrate table WT configured to support a substrate W. The illuminationsystem IL is configured to condition the radiation beam B before it isincident upon the patterning device MA. The projection system isconfigured to project the radiation beam B (now patterned by the maskMA) onto the substrate W. The substrate W may include previously formedpatterns. Where this is the case, the lithographic apparatus aligns thepatterned radiation beam B with a pattern previously formed on thesubstrate W. In this embodiment, the pellicle 15 is depicted in the pathof the radiation and protecting the patterning device MA. It will beappreciated that the pellicle 15 may be located in any required positionand may be used to protect any of the mirrors in the lithographicapparatus.

The radiation source SO, illumination system IL, and projection systemPS may all be constructed and arranged such that they can be isolatedfrom the external environment. A gas at a pressure below atmosphericpressure (e.g. hydrogen) may be provided in the radiation source SO. Avacuum may be provided in illumination system IL and/or the projectionsystem PS. A small amount of gas (e.g. hydrogen) at a pressure wellbelow atmospheric pressure may be provided in the illumination system ILand/or the projection system PS.

The radiation source SO shown in FIG. 1 is of a type which may bereferred to as a laser produced plasma (LPP) source. A laser 1, whichmay for example be a CO₂ laser, is arranged to deposit energy via alaser beam 2 into a fuel, such as tin (Sn) which is provided from a fuelemitter 3. Although tin is referred to in the following description, anysuitable fuel may be used. The fuel may for example be in liquid form,and may for example be a metal or alloy. The fuel emitter 3 may comprisea nozzle configured to direct tin, e.g. in the form of droplets, along atrajectory towards a plasma formation region 4. The laser beam 2 isincident upon the tin at the plasma formation region 4. The depositionof laser energy into the tin creates a plasma 7 at the plasma formationregion 4. Radiation, including EUV radiation, is emitted from the plasma7 during de-excitation and recombination of ions of the plasma.

The EUV radiation is collected and focused by a near normal incidenceradiation collector 5 (sometimes referred to more generally as a normalincidence radiation collector). The collector 5 may have a multilayerstructure which is arranged to reflect EUV radiation (e.g. EUV radiationhaving a desired wavelength such as 13.5 nm). The collector 5 may havean elliptical configuration, having two ellipse focal points. A firstfocal point may be at the plasma formation region 4, and a second focalpoint may be at an intermediate focus 6, as discussed below.

The laser 1 may be separated from the radiation source SO. Where this isthe case, the laser beam 2 may be passed from the laser 1 to theradiation source SO with the aid of a beam delivery system (not shown)comprising, for example, suitable directing mirrors and/or a beamexpander, and/or other optics. The laser 1 and the radiation source SOmay together be considered to be a radiation system.

Radiation that is reflected by the collector 5 forms a radiation beam B.The radiation beam B is focused at point 6 to form an image of theplasma formation region 4, which acts as a virtual radiation source forthe illumination system IL. The point 6 at which the radiation beam B isfocused may be referred to as the intermediate focus. The radiationsource SO is arranged such that the intermediate focus 6 is located ator near to an opening 8 in an enclosing structure 9 of the radiationsource.

The radiation beam B passes from the radiation source SO into theillumination system IL, which is configured to condition the radiationbeam. The illumination system IL may include a facetted field mirrordevice 10 and a facetted pupil mirror device 11. The faceted fieldmirror device 10 and faceted pupil mirror device 11 together provide theradiation beam B with a desired cross-sectional shape and a desiredangular distribution. The radiation beam B passes from the illuminationsystem IL and is incident upon the patterning device MA held by thesupport structure MT. The patterning device MA reflects and patterns theradiation beam B. The illumination system IL may include other mirrorsor devices in addition to or instead of the faceted field mirror device10 and faceted pupil mirror device 11.

Following reflection from the patterning device MA the patternedradiation beam B enters the projection system PS. The projection systemcomprises a plurality of mirrors 13, 14 which are configured to projectthe radiation beam B onto a substrate W held by the substrate table WT.The projection system PS may apply a reduction factor to the radiationbeam, forming an image with features that are smaller than correspondingfeatures on the patterning device MA. A reduction factor of 4 may forexample be applied. Although the projection system PS has two mirrors13, 14 in FIG. 1, the projection system may include any number ofmirrors (e.g. six mirrors).

The radiation sources SO shown in FIG. 1 may include components whichare not illustrated. For example, a spectral filter may be provided inthe radiation source. The spectral filter may be substantiallytransmissive for EUV radiation but substantially blocking for otherwavelengths of radiation such as infrared radiation. Indeed, thespectral filter may be a pellicle according to any aspect of the presentinvention.

FIG. 2 shows a schematic depiction of a pellicle in accordance with thepresent invention. The pellicle 15 comprises a metal-silicide-nitride ornitridated silicon layer 16 sandwiched between capping layers 17.

The term “EUV radiation” may be considered to encompass electromagneticradiation having a wavelength within the range of 4-20 nm, for examplewithin the range of 13-14 nm. EUV radiation may have a wavelength ofless than 10 nm, for example within the range of 4-10 nm such as 6.7 nmor 6.8 nm.

FIGS. 3a to c shows a schematic of the steps used in selecting thecorrect compensating layer for a given pellicle. A pellicle P issubjected to the conditions in a lithography apparatus and the change intransmissivity of the pellicle P is measured. The pellicle P is shown asbeing comprised of a single layer in the schematic drawings, but this isfor the sake of simplicity and it will be appreciated that the pellicleP may comprise a pellicle stack. As such, the pellicle P may compriseone or more layers. Once the change in transmissivity of a givenpellicle P has been measured, the measured change is transmissivity isused to select a compensating layer CL material which most closelydisplays the opposite change in transmissivity. This information is thenused to create an updated pellicle P comprising the compensating layerCL. The updated pellicle may then undergo the same testing to refine thenature of the compensating layer CL. As shown in FIG. 3c , thecompensating layer CL has been increased in thickness, but it will beappreciated that this is not the only change which is possible and otherpossible changes include providing a thinner compensating layer CL,moving the compensating layer to a different part of the pellicle P, oreven changing the material comprising the compensating layer CL.

As an example, a MoSiN_(x) pellicle was exposed to EUV radiation for 20hours at 580° C. under a pressure of 3 Pa of hydrogen and it was foundthat the transmissivity of the pellicle increased by around 1%. It isbelieved that this is due to the surface being terminated with siliconoxynitride that is susceptible to photonic etching and thereby becomingthinner. Another MoSiN_(x) pellicle was coated with a layer of boron andtested at around 540° C. under a pressure of 3 Pa of hydrogen for 20hours. This resulted in around a 1% decrease in the transmissivity ofthe pellicle. As such, the boron layer counteracted the changes intransmissivity caused by the etching of the silicon oxynitride. Thus,the thickness of the boron layer could be altered in order to result ina thinner layer of boron oxide being formed such that the change intransmissivity of the pellicle was closer to 0%.

FIG. 4 depicts a cross-section of a membrane assembly manufactured inaccordance with an existing method. The membrane assembly 18 comprises aborder 19 which is manufactured from a planar substrate. Any suitableplanar substrate may be used, but a silicon border will be discussedherein. A thermal oxide layer 20 is provided on the border 19. In theexample, the thermal oxide layer 20 is a silicon oxide layer. A membranelayer 21 is provided on the oxide layer 20. The membrane layer 21comprises molybdenum silicon nitride, although other materials may beused. A TEOS layer 22 is provided on the membrane layer 21. The TEOSlayer may subsequently be processed to form a silicon oxide layer.Alternatively, instead of thermal oxide or TEOS, layers 20 and/or 22 maybe a SiN layer having a thickness up to 10 nm, for example in a rangefrom 1 to 5 nm.

During manufacture, a TMAH based etchant is used to etch away the innerregion of the planar substrate. In order to ensure that the requiredamount of the planar substrate has been removed, the etching step isallowed to proceed for long enough that the etchant begins to etch awayat the thermal oxide layer. Although the TMAH based etchant etchessilicon oxide at a lower rate than silicon, due to the need to ensurethat the inner region of the planar substrate, the etching continues andnotches are formed around the edge of the thermal oxide layer. Theetching step may take more than an hour, and the over etching can takeplace for around a minute. As such, in order to ensure that the etchantdoes not etch into the membrane layer, the thermal oxide layer needs tobe relatively thick, which may be 50 nm or more. Since the thermal oxidelayer is compressive, this may induce wrinkling of the membrane, whichcan weaken the assembly. In addition, the additional thickness of thethermal oxide layer may result in lower EUV transmissivity of themembrane assembly.

FIG. 5a depicts a schematic cross section of a membrane assemblymanufactured according to the present invention. FIG. 5a depicts themembrane assembly at an early stage of manufacture. The same numeralsare used for the features corresponding to those in FIG. 4. In contrastto the membrane assembly of FIG. 4, the membrane assembly 18 depicted inFIG. 5a additionally comprises a buried oxide layer 24 and a furtherlayer 25 located between the border 19 and the membrane layer 21. Thefurther layer 25 may be a silicon layer.

During manufacture, as with the method depicted in FIG. 4, a TMAHetchant is used to bulk etch the inner region of the planar substrate toprovide a border 19. The buried oxide layer 24 serves the same purposeas the thermal oxide layer 20 in FIG. 4 in that it resists the etchantused to etch the silicon from the inner region of the planar substrate.As shown in FIG. 5a , this will result in notches 23 being formed aroundthe edge of the buried oxide layer 24.

In a following step shown in FIG. 5b , a different etchant, such as BOE,is used to remove an inner portion of the buried oxide layer 24. Sincethe overlying further layer 25 comprising silicon is resistant toetching by BOE, the over etch of the buried oxide layer 24 is nottransferred to the further layer 25. In this way, these layers serve asetch homogenisation layers.

Subsequently, as shown in FIG. 5c , a further etching step may becarried out in which an etchant, such as TMAH etchant, is used to removethe inner region of the further layer 25. Since the further layer 25 ismuch thinner than the planar substrate, the time to which the thermaloxide layer 20 is exposed to etchant is reduced from more than one hourto a few minutes. This drastically reduces the potential for overetching of the thermal oxide layer 20 since the potential over etchingonly lasts a few seconds and thereby allows the thermal oxide layer 20to be thinner than would be the case in the existing method ofmanufacture. For example, the thermal oxide thickness could be reducedfrom 50 nm or more to less than 50 nm.

The buried oxide layer and the thermal oxide layer may be produced inthe same way or different ways and the exact method of producing theselayers is not particularly limiting on the invention. The membrane layermay comprise multiple layers. For example, the membrane layer maycomprise a molybdenum silicon nitride layer sandwiched between twomolybdenum silicide layers.

The method according to the eleventh aspect of the present inventionprovides for a membrane assembly in which over-etching is reduced,resulting in a stronger and more consistent membrane assembly. Thismethod also reduces the stress mismatch between layers of the membraneassembly as sacrificial oxide layers can be made thinner without therisk of over etching. This reduces compressive forces on the assemblyand reduces the risk of wrinkling. In addition, since the planarsubstrate, buried oxide layer and overlying silicon layer can beprovided as a silicon-on-insulator type wafer (SOI), this can reduce thenumber of manufacturing steps prior to etching, which may reduce costsand the risk of particulate contamination.

The membrane assemblies may be used as pellicles, preferably in EUVlithography machines, but can also find application as spectral purityfilters.

FIG. 6 schematically depicts the method according to the thirteenthaspect of the present invention. There is provided a stack 26 comprisinga planar substrate 27, an optional thermal oxide layer 28 at leastpartially surrounding the planar substrate 27, a membrane layer 29 atleast partially surrounding the thermal oxide layer 28, and aboron-doped TEOS layer 30. Prior to annealing the membrane layer 29 issubstantially free of boron. The pattern in boron-doped layer 30 isintended to indicate the presence of boron atoms in the layer and howthis passes into the membrane layer 29 after annealing.

In an annealing step, the stack is heated to a temperature sufficient toallow boron in the boron-doped TEOS layer 30 to diffuse into themembrane layer 29. This results in a boron-enriched membrane layer 29and a reduction in the amount of boron in the boron-doped TEOS layer 30.It will be appreciated that not all of the boron may diffuse into themembrane layer 29, and the exact amount of boron which diffuses into themembrane layer 29 can be controlled by adjusting the temperature andduration of the annealing step, as well as the concentration of theboron in the boron-doped TEOS layer 30. The membrane layer may compriseat least one of silicon, molybdenum silicide, and molybdenum siliconnitride.

The boron-doped membrane containing assemblies are eminently suitablefor use as pellicles in EUV lithography machines as well as use asspectral purity filters.

Although specific reference may be made in this text to embodiments ofthe invention in the context of a lithographic apparatus, embodiments ofthe invention may be used in other apparatus. Embodiments of theinvention may form part of a mask inspection apparatus, a metrologyapparatus, or any apparatus that measures or processes an object such asa wafer (or other substrate) or mask (or other patterning device). Theseapparatus may be generally referred to as lithographic tools. Such alithographic tool may use vacuum conditions or ambient (non-vacuum)conditions.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. The descriptions above are intended to beillustrative, not limiting. Thus it will be apparent to one skilled inthe art that modifications may be made to the invention as describedwithout departing from the scope of the claims set out below.

1. A pellicle for a lithographic apparatus, wherein the pelliclecomprises nitridated metal silicide.
 2. The pellicle according to claim1, wherein the nitridated metal silicide has the formulaM_(x)(Si)_(y)N_(z), wherein x≤y≤2x, and 0<z≤x. 3.-5. (canceled)
 6. Thepellicle according to claim 1, wherein the metal is selected from: Ce,Pr, Sc, Eu, Nd, Ti, V, Cr, Zr, Nb, Mo, Ru, Rh, La, Y, or Be. 7.(canceled)
 8. The pellicle according to claim 1, wherein the atomicconcentration of nitrogen in the metal-silicide-nitride is less thanaround 25%.
 9. The pellicle according to claim 1, wherein the pelliclefurther comprises at least one capping layer. 10.-12. (canceled)
 13. Thepellicle according to claim 9, wherein the capping layer comprisesruthenium, boron, metal boride, boron carbide, and/or boron nitride. 14.A method of manufacturing a pellicle for a lithographic apparatus, themethod comprising nitriding a metal silicide or silicon substrate forthe pellicle.
 15. (canceled)
 16. The method according to claim 14,wherein the metal silicide or silicon substrate is a metal silicide orsilicon film. 17.-19. (canceled)
 20. The method according to claim 14,where the metal of the metal silicide or silicon substrate is selectedfrom: Ce, Pr, Sc, Eu, Nd, Ti, V, Cr, Zr, Nb, Mo, Ru, Rh, La, Y, or Be.21. (canceled)
 22. The method according to claim 14, wherein thesubstrate comprises silicon.
 23. A pellicle for a lithographic apparatusobtainable or obtained by the method according to claim
 14. 24.-25.(canceled)
 26. An assembly for a lithographic apparatus comprising thepellicle according to claim 1, a frame for supporting the pellicle and apatterning device attached to the frame.
 27. A pellicle for alithographic apparatus, the pellicle comprising at least onecompensating layer selected and configured to counteract changes intransmissivity of the pellicle upon exposure to EUV radiation. 28.-38.(canceled)
 39. A method of manufacturing a membrane assembly for EUVlithography, the method comprising: providing a stack comprising: aplanar substrate, wherein the planar substrate comprises an inner regionand a border region around the inner region; at least one membranelayer; an oxide layer between the planar substrate and the at least onemembrane layer; and at least one further layer between the planarsubstrate and the at least one membrane layer; and selectively removingthe inner region of the planar substrate, such that the membraneassembly comprises: a membrane formed at least from the at least onemembrane layer; and a border holding the at least one membrane layer,the border comprising at least a portion of the planar substrate, the atleast one further layer, and the oxide layer situated between the atleast a portion of the planar substrate and the at least one membranelayer. 40.-48. (canceled)
 49. A membrane assembly for EUV lithography,the membrane assembly comprising: a membrane formed from at least onelayer comprising molybdenum silicon nitride; and a border holding themembrane, wherein the border is formed from a planar substratecomprising an inner region and a border region around the inner region,wherein the border is formed by selectively removing the inner region ofthe planar substrate, and wherein the assembly comprises a buried oxidelayer, a silicon layer, and a thermal oxide layer between the border andthe membrane.
 50. (canceled)
 51. A method of preparing a stack, themethod comprising: providing a planar substrate, a membrane layer, and atetraethyl orthosilicate layer as a stack, and annealing the stack,wherein the tetraethyl orthosilicate layer includes boron such that atleast a portion of the boron from the tetraethyl orthosilicate layerdiffuses into the membrane layer during annealing. 52.-55. (canceled)56. A stack comprising a planar substrate and a membrane layer, whereinthe membrane layer is doped with boron.
 57. (canceled)
 58. The stackaccording to claim 56, wherein the membrane layer comprises at least oneselected from: silicon, molybdenum silicide, or molybdenum siliconnitride.
 59. The stack according to claim 56, further comprising athermal oxide layer between the planar substrate and the membrane layer.60. The stack according to claim 56, further comprising aboron-containing TEOS layer at least partially surrounding the membranelayer.